1. Field of the Invention
The invention relates to a multi-power domain operational amplifier and a voltage generator using the same.
2. Description of the Related Art
Because of the properties of semiconductor elements, many applications need a set of a positive reference voltage and a negative reference voltage, which are not affected by the temperature and are about +5 volts and −5 volts, respectively. The +5 volts and −5 volts approach the voltage withstanding upper bound (6 volts) of the medium voltage element. In the industry, a bandgap reference circuit is typically utilized to generate a zero temperature coefficient reference voltage of about 1.2 volts, and boost and buck operations are performed through a regulator based on the zero temperature coefficient reference voltage. Thus, various reference voltages for various applications can be generated.
FIG. 1 is a circuit diagram showing an example of a conventional voltage generator 10. Referring to FIG. 1, the voltage generator 10 includes a unity gain buffer 12, a first regulator 14 and a second regulator 16. In FIG. 1, the operation voltage VDD is equal to 3 volts, for example, and the zero temperature coefficient reference voltage Vref generated by the bandgap reference circuit is equal to 1.2 volts, for example. Boost and buck operations may be performed on the zero temperature coefficient reference voltage Vref by utilizing the first regulator 14 and the second regulator 16 according to the resistance relationship: (R1+R2)/R1=5/1.2 so that the positive reference voltage VoutP=5 volts and the negative reference voltage VoutN=−5 volts can be obtained. Because the second regulator 16 takes the ground voltage (0 volts) as the reference point, the power domain of its internal operational transductance amplifier (OTA) 18 needs to range from VDD to −2 VDD (i.e., from 3 volts to −6 volts), which exceeds the voltage withstanding restriction of the medium voltage element. So, the high voltage element has to be adopted. Consequently, the poor element property of the high voltage element reduces the overall circuit behavior, and occupies a lot of layout area.
FIG. 2 is a circuit diagram showing another example of a conventional voltage generator 20. Referring to FIG. 2, the voltage generator 20 includes a unity gain buffer 22, a first regulator 24, a second regulator 26 and a third regulator 28. In FIG. 2, the operation voltage VDD is equal to 3 volts, for example, and the zero temperature coefficient reference voltage Vref generated by the bandgap reference circuit is equal to 1.2 volts, for example. The zero temperature coefficient reference voltage Vref utilizes the first regulator 24 to perform a boosting operation according to the resistance relationship and thus obtain the positive reference voltage VoutP=5 volts. In addition, the zero temperature coefficient reference voltage Vref utilizes the second regulator 26 to perform a bucking operation with the ground voltage (0 volts) serving as the reference point to obtain −Vref=−1.2 volts first, and then utilizes the third regulator 28 to perform two bucking operations to obtain the negative reference voltage VoutN=−5 volts. With the cascaded two stages of regulator structures 26 and 28, the power domain of the second regulator 26 ranges from VDD to −VDD (i.e., from 3 volts to −3 volts), and the power domain of the third regulator 28 ranges from GND to −2 VDD (i.e., from 0 volts to −6 volts), wherein the power domains are kept within the voltage withstanding range of the medium voltage element and the use of the high voltage element can be avoided. In the structure of the voltage generator 20, however, an additional stage of regulators may make the output voltage have an offset and be affected by the power noise.
In addition, in the process of transforming the positive voltage into the negative voltage, the voltage generators 10 and 20 need to use the unity gain buffers 12 and 22, respectively, so that the zero temperature coefficient reference voltage Vref possesses the driving ability to provide the current output. However, using the unity gain buffers 12 and 22 increases the circuit complexity and the layout area, and also increases the offset of the output voltage and the influence of the power noise.